Audio systems utilizing amplifier circuits for amplifying audio source signals in order to drive audio devices have been widely used in various electronic products. In a typical application circuit, as shown in FIG. 1, an audio amplifier chip 100 is used to drive an earphone 104 or a loudspeaker 106 based on an audio source signal 102. The audio source signal 102 is fed into the chip 100 through an input coupling capacitor 108 and an input resistor 110, and the chip 100 includes two operational amplifiers 112 and 114, with the inverting and non-inverting inputs 116 and 118 of the operational amplifier 112 connected to the input resistor 110 and a reference signal Vref, respectively, so as to generate an output signal Vo+ at the output 120 of the operational amplifier 112 to drive the earphone 104. To provide the reference signal Vref, a voltage divider composed of two resistors 132 and 134 are connected in series to a supply voltage VDD by a power input 136, in such a way that at a node 138 the reference signal Vref is generated, for example equal to VDD/2, to connect to the non-inverting inputs 118 and 128 of the operational amplifiers 112 and 114. A capacitor 140 may be coupled to the power input 136 to stable the supply voltage VDD for the chip 100. A bypass capacitor 142 is coupled to the node 138 to be charged to the reference voltage VDD/2, and the capacitance CB of the bypass capacitor 142 determines the charge rate of the reference signal Vref up to VDD/2 and the discharge rate of the voltage on the node 138. The operational amplifier 112, the input resistor 110, and a feedback resistor 144 connected between the output 120 and the inverting input 116 of the operational amplifier 112 constitute a well-known amplifier, and in which the feedback resistor 144 and the input resistor 110 are both outside of the chip 100 for adjusting the gain of the amplifier. However, the feedback resistor 144 and the input resistor 110 may be integrated in the chip 100 instead. The other amplifier is constituted by the operational amplifier 114, an input resistor 122 and a feedback resistor 124, and has a unit gain, for inverting the output signal Vo+ to generate another output signal Vo−. The output signals Vo+ and Vo− constitute a pair of differential output signals for driving the loudspeaker 106. The chip 100 further comprises a bias control circuit 146 to generate two control signals 152 and 154, based on a sleep or shutdown signal SHUTDOWN originated from the system control unit and a select signal or bridge-tied-load signal SEL/ BTL originated from the earphone socket, for the control of enabling and disabling the operational amplifiers 112 and 114.
During power-off, owing to the different discharge rates of the input coupling capacitor 102 and the bypass capacitor 142, an instant variation may occur in the signal at the output 120 of the chip 100, resulting in an unfavorable noise, referred to ‘pops’, emanated from the earphone 104 or loudspeaker 106. To remove this pops generation, a proposed solution is to provide two individual power supplies for the amplifier circuit of the chip 100. However, introducing a second power supply will have a higher system complexity and cost. In U.S. Pat. No. 5,939,938 to Kalb et al., switching the gain of or the input to the amplifier and controlling the discharge rate of the bypass capacitor are used during power-off to suppress the pops generation. The control signal for switching the gain of or the input to the amplifier and controlling the discharge rate of the bypass capacitor is further delayed in U.S. Pat. No. 6,346,854 to Heithoff, for a more precise control. However, these proposed arts eliminate only part of the factors causing power-off pops, and the improvement is not perfect enough. Even though the supply voltage drops down rapidly during power-off, the operational amplifier is still operated in a common mode before it is disabled, and thus the over-driving will result in instant variations of the output signal to generate pops. A detector circuit to detect the voltage difference between the input and output of a voltage regulator is proposed in U.S. Pat. No. 4,181,895 to Yoshida, by which a mute circuit is triggered to have the output of the amplifier silent when the voltage difference is lower than a predetermined threshold, and thus no pops are generated. On the other hand, after disabling the operational amplifier, due to the output signal dropping down slower than the supply voltage, it may force the body diode parasitic to the output stage transistor of the operational amplifier forward-biased, thereby providing another conductive pass between the drain and source of the output stage transistor to induce instant variations in the output signal and to generate pops accordingly. In U.S. Pat. No. 6,525,594 to Fugate et al., the body of the output stage transistor of the operational amplifier circuit is connected with a switch switching to a power input or a current path of the output stage transistor to prevent the body voltage from lower than the voltage on the current path. Alternatively, in U.S. Pat. No. 6,542,024 to Somayajula, a voltage is coupled to the body of the output stage transistor of the operational amplifier when the supply voltage drops down, to prevent the body voltage from lower than a predetermined threshold.